In general, an image reproduction apparatus reproduces images on an image-carrying medium by transferring developer containing toner particles onto the medium in relation to a given image. Such transfer is typically achieved through the use of a developing unit which places toner or developer on the image-carrying medium via a photoreceptor drum. To accomplish image transfer, the photoreceptor drum surface is first prepared by an electrophotographic image process to selectively accept toner in relation to the image. The developing unit then applies toner onto the photoreceptor drum via a developing roller. Toner representing the desired image on the photoreceptor drum is transferred onto an image-carrying medium such as paper. Further processing of the paper, for example, the application of heat, serves to permanently adhere the toner particles to the paper.
High resolution images may be developed with a liquid carrier based developer, also known as a wet developer. The wet developer contains liquid carrier and micro toner particles suspended in the liquid carrier. To develop a high resolution image on an image-carrying medium, the liquid carrier based developer is applied from a developing roller to the photoreceptor drum according to an image pattern. The applied developer on the photoreceptor drum forms a layer having a depth ranging from approximately 30 .mu.m to approximately 200 .mu.m. The toner particles in the liquid carrier are ultimately transferred onto an image-carrying medium such as paper by applying a predetermined voltage to the transfer roller which holds the image-carrying medium. During this transfer, the liquid carrier facilitates the electrophoresis of toner particles by serving as a conduit between the photoreceptor drum surface and an image-carrying medium. Since the toner particle size in the liquid carrier based developer is approximately 0.1 to 0.5 .mu.m in diameter in comparison to approximately a 5 .mu.m toner particle in a dry developer, the above-described wet developer generally produces higher resolution images.
During the toner transfer onto an image-carrying medium, the amount of the liquid carrier for a particular image forming surface affects the image quality on that image-carrying medium. If the liquid carrier amount is excessive, as shown in FIG. 1A, for example, a portion of toner particles 24 is washed away from a body of toner particles 22 representing a line on a image-carrying medium 20. The resulted image is generally blurred. When the line 22 is seen in a cross sectional view taken at A--A, FIG. 1B illustrates the washed away toner particles 24 beside the line 22. On the other hand, if the liquid carrier amount is insufficient, an adequate amount of toner particles may not be transferred onto the image-carrying medium and the image generally suffers from light or even white spots. To improve the above-described undesirable images, the amount of liquid carrier on the photoreceptor drum needs to be adjusted based upon the image-carrying surface. For reducing the excess liquid carrier, the liquid carrier reduction means is used.
The excess carrier liquid reduction means includes a) a reverse roller also known as a squeeze roller, b) an air knife and c) a corona discharger. These means are generally located adjacent to a developing roller and a photoreceptor drum and remove an excess amount of carrier liquid after the liquid carrier based developer is applied to the photoreceptor drum. More particularly, for example Japanese Patent 63-178277 discloses that the squeeze roller is placed at a predetermined distance from the photoreceptor drum so that the squeeze roller physically contacts a portion of the carrier liquid layer placed on the photoreceptor drum. As the photoreceptor drum rotates with respect to the squeeze roller, the squeeze roller removes an excess amount of the liquid carrier from the developer layer. During this removal, the squeeze roller itself may also rotate to further control the amount of removal. In addition, the speed and/or the direction of rotation of the squeeze roller further control the amount of the liquid carrier removal.
In contrast to the above-described squeeze roller, the air knife does not physically contact the excess liquid carrier layer. After the liquid carrier based developer is applied onto the photoreceptor drum surface, the predetermined pressurized air is directed towards the excess liquid carrier surface while the photoreceptor drum is rotated and an excess amount of liquid carrier is removed by the rapid air flow.
As a third device for removing the excess liquid carrier, the corona discharger is also located at a predetermined distance from the photoreceptor drum and does not generally contact the excess liquid carrier layer. After a liquid carrier based developer is applied onto the photoreceptor drum, the corona discharger removes a predetermined excess amount of liquid carrier by discharging an ion beam towards the excess liquid carrier surface on the rotating photoreceptor drum. Since the ion beam has the same polarity as the liquid carrier, due to the repulsion, the liquid carrier is removed from the rotating photoreceptor drum surface.
In summary, any one of the above-described excess carrier liquid reduction means removes a fixed predetermined amount of carrier liquid for a single image-carrying medium. In order to render an optimal image, a right amount of liquid carrier is important for the reproduction process using a liquid carrier based developer.
In addition to the above-described liquid carrier adjustment problems, another disadvantage of using a liquid carrier based developer is that the straight edges in the rendered image are often deformed as illustrated in FIG. 2A. Referring to FIG. 2B, a cross section taken at B--B illustrates a distorted or collapsed straight edge 26. In other words, the toner particles in the vicinity of the straight edges are not held together to form a straight line. As a result, the straight line 22 appears to zigzag. The collapsed straight edge 26 generally occurs in any direction with respect to images or characters, but it appears that the collapsed edges occur more often in the trailing side of the moving direction as the image is rendered on the image-carrying medium. It is not clear as at which step of the reproduction process these toner particles near the straight edge break loose and collapse on the image-carrying medium surface.
In recent years, using a wet developer, high resolution images are in demand on various types of paper. These various types of paper present different image forming surfaces whose characteristics include smoothness and liquid absorbency. These surface characteristics affect the images produced by the liquid carrier based developer. Since each of these surface characteristics has a spectrum of degrees, there is a large number of combinations to define an image forming surface. However, a generalization of these paper types may be made in relation to the images produced by a liquid carrier based developer. In general, relatively absorbent and rough paper requires a larger amount of liquid carrier based developer than relatively unabsorbent and smooth paper. This is because liquid carrier is more easily absorbed by the paper so that more developer is necessary to ascertain a transfer of a sufficient amount of toner particles onto an image-carrying medium. On the other hand, a smooth or coated paper does not require as much wet developer since the image-carrying surface does not absorb the liquid carrier during toner particle transfer.
In contrast to a single predetermined paper type, the copying technology requires a method and an apparatus to accommodate various paper types in a single reproduction machine using a high-resolution liquid carrier based developer. Although the above-described duplicator system such as Indigo EP-100 is commercially available, the system is generally expensive and physically large due to the use of an intermediate transfer roller. The intermediate transfer roller enables the use of various type of image-carrying media since it generates a toner-containing film layer. When this film layer is transferred onto the image-carrying medium, it is not generally affected by the image forming surface characteristics to form an optimal image. However, without such an expensive intermediate transfer roller, as described above, the wet developer presents at least two types of problems including an excess liquid carrier removal and toner particle collapses. The technology requires commercially viable solutions to these problems so as to reproduce inexpensive high-resolution images using a wet developer.